The present invention relates generally to composite structures, those comprising two or more dissimilar materials, and such structures are well-known and in widespread use. One common type of composite structure is a ferroconcrete girder made from one or more steel beams with a concrete overlayment. The steel portion of the composite structure is situated toward the bottom of the structure whereas the concrete lies atop the steel. This arrangement takes advantage of the structural properties of the steel and concrete and makes for a cost-effective structure which has an adequate factor of safety.
Further with regard to ferroconcrete composite structures, the steel portion of the structure is commonly in the form of an "I" beam and the concrete is cast upon the I-beam with the two materials forming a homogeneous or integral unit once the concrete has cured. The steel forms a "tensile layer" whereas the concrete forms a "compressive layer." That is, it is desirable to fabricate the ferroconcrete structure such that most of the concrete lies above the neutral plane of the structure so that the concrete is substantially under compression due to the dead and live loads on the composite structure. On the other hand, the steel is located primarily below the neutral plane so that the steel can absorb the tensile stresses which are incurred by the composite structure when the structure is subjected to the dead and live loads. As well known to those skilled in the art, the I-beam of a ferroconcrete composite structure is not typically subjected solely to tensile stresses, but the phrase "tensile layer" will be used to refer to the I-beam or like elements in other composite structures for the sake of brevity.
The foregoing is quite well known to those skilled in the art, and the particular arrangement of steel and concrete in a ferroconcrete composite structure is chosen primarily due to the weakness of concrete in tension and due to the fact that concrete makes a superior overlayment and is sufficiently strong in compression.
A composite structure of the type discussed above should be distinguished from reinforced concrete and the like. Reinforced concrete is comprised primarily of concrete but includes one or more slender members typically made of steel which are held in tension by the concrete. That is, the concrete is compressed by the steel cable or rods whereas the rods are held in tension by the concrete, and this compression of the concrete tends to overcome any deliterious effects caused by placing the concrete in tension.
In contrast to reinforced concrete, the present invention relates to a true composite structure such as a ferroconcrete girder. In a composite structure of the type contemplated by the present invention, the steel reinforcing layer is capable of withstanding bending stresses and does not primarily function to place a portion of the concrete in compression as was the case in reinforced concrete structures.
As well known to those skilled in the art, composite structures are not limited to ferroconcrete girders. Ferroconcrete composite structures can be used for other structural members and the present invention is not limited to a ferroconcrete girder, i.e. a horizontal main structural member that supports vertical loads.
Furthermore, other materials can be used for fabricating composite structures and are contemplated by the present invention. Wood and laminated wood can be used for a tensile layer, for example, and, in fact, wood can also be used for the compressive layer. The present invention is not limited to any particular material or combination of materials as is clear to those skilled in the art of the fabrication of structural members. However, for the sake of brevity, and only as an example, the present description of the prior art and the detailed description of the invention will be limited to ferroconcrete structues.
As noted above, the present invention is related to composite structures, but more particularly it is related to "prestressed" composite structures. It is well known in the art to "prestress" a composite structure to take better advantage of the properties of the materials. For example, it is well known to prestress a steel beam to produce a convex surface and a concave surface in the beam and then cast the concrete layer on the convex surface of the beam. Once the concrete has cured, the bending moment is removed and the concrete layer is subjected to compression while the uppermost layer or flange of the steel beam is subjected to tension and the lower flange of the beam is held in compression. The concrete layer, or "compressive" layer, in effect "locks in" the stresses in the steel beam formerly induced by the bending moment. With the upper portion of the steel beam in tension and the lower portion in compression the beam is prestressed and is better able to accommodate dead and live loads. That is, the concrete which forms the compressive layer absorbs a portion of the dead and live loads as it compresses, but the concrete also serves to maintain the prestress in the steel beam so that it can better absorb the tensile stresses at the bottom flange induced by the dead and live loads. The end result is that the cross-section of the steel beam can be reduced while at the same time the applicable factor of safety is met. Clearly, this reduction in the cross-section of the steel beam results in a considerable cost savings. Alternatively, the cross-section of the steel beam can be maintained and the prestressed composite structure can withstand larger loads than a visually similar structure which has not been prestressed.
As well known in the art of ferroconcrete fabrication, the compressive and tensile layers, the concrete slab and steel beam, must be bound together so as to act as a single integral structural unit. This can be accomplished either by securely bonding the concrete to the steel beam or by using a shear connector of some type. Shear connectors are also well known in the art, one type being a stud which projects from the upper flange of the steel beam and around which the concrete is cast. Shear stresses are transmitted through the studs from one layer of the composite structure to the other. Other types of shear connectors are contemplated by the present invention, such as a spiral device which is welded to the top flange of the beam.
Various methods for making prestressed composite structures have been proposed. One method for making composite structures is represented by the method shown in U.S. Pat. No. 4,006,523, issued to Mauquoy. In this method, transmission elements are securely attached to the bottom flange of the steel beam at opposite ends of the beam. High strength wires or cables pull the transmission elements toward one amother to bend the beam and encasing concrete is cast around the beam, transmission elements and cable.
Several shortcomings are perceived with this method for prestressing a composite structure. First, the transmission elements must be securely attached to the bottom portion of the beam using, for example, a welding process. This step is time consuming and expensive. Secondly, the cables and transmission elements must produce very large forces in order to sufficiently bend the beam prior to pouring the encasing concrete. This is due to the limited moment arm that the transmission elements provide. The very large forces induced in the cable and transmission elements poses a safety problem.
The method as shown in U.S. Pat. No. 4,006,523 also requires that there be sufficient clearance below the beam for the welding and encasing processes. In some cases, this clearance is not available such as in bridge construction where overhead clearance is critical.
Additionally, this method of prestressing a composite structure would be difficult if not impossible to implement with preexisting structures. For example, on occasion it is desirable to increase the load-carrying capability of a girder which have been in operation for some time. It would be desirable to prestress the girder by removing and recasting the concrete without having to remove the girder from the bridge. The method represented by the method shown in U.S. Pat. No. 4,006,523 clearly suffers from shortcomings when preexisting structures are involved: the clearance problem discussed above might preclude the use of this method altogether, and it might be very difficult in some cases to adequately access the bottom flange of the beam to weld the transmission elements in place.
Still another prestressing method that has been suggested includes simply supporting the steel beam at its ends, and allowing the center portion of the beam to sag between the support points. Forms are attached to the beam and concrete is cast such that it is in contact with the bottom flange of the beam. The weight of the form and the concrete causes the beam to sag even further. The bending moment created by the weight of the beam, form and concrete induces a prestress in the beam and the composite structure.
When the concrete has sufficiently cured, the composite structure is flipped or rotated so that the concrete is on the top side of the composite structure, the concrete forming an overlayment for the structure. The concrete locks the prestress into the structure and the dead and live loads applied to the structure are more easily handled. That is, the dead and live loads cause the concrete to compress and the steel beam to bend in a direction opposite to the sag or bend which was initially preset into the composite structure. The prestresses which were induced and locked into the steel beam are opposite to the stresses induced in the beam due to the dead and live loads and therefore the prestresses act to counter the stresses due to the loads on the composite structure and particularly on the steel beam.
This method for making a prestressed composite structure also possesses several shortcomings. As noted above, once the concrete has cured, the composite structure must be rotated prior to use. Even if such composite structures are fabricated in a manufacturing plant, this flipping procedure is difficult and expensive since the composite structure is typically quite massive and unwieldy.
Furthermore, this method of casting the concrete on the underside of the inverted beam cannot easily be used with pre-existing structures. For example, if this method were attempted to be used to increase the load carrying capability of a bridge girder, the bridge girder would have to be removed from the bridge and reworked or prestressed. The concrete casting process clearly would not be accomplished while the beam is in place in the bridge structure since the resulting composite structure could not be flipped without removing it from the bridge.
The present invention is directed to the shortcomings noted above with respect to the prior art methods. The present invention is a method for prestressing a composite structure which does not require the attachment of transmission elements or the like to the tensile layer, the steel beam in a ferroconcrete composite structure. Furthermore, the present invention does not require that the resulting composite structure be flipped following the engagement of the compressive layer with the tensile layer. On the contrary, the present method is quite simple to use and, in fact, can be utilized to rehabilitate preexisting structures without requiring the removal of the structures or structural components from the main body of the structure. In other words, the method can be used in situ.